A light receiving element is an element used for converting an optical signal into an electrical signal, and used in various fields. In the field of optical discs such as CD (compact disc) and DVD (digital versatile disc), in particular, a light receiving element is important as a key device of an optical head device (optical pickup) which reads and writes a signal recorded on an optical disc. As a higher performance and a higher integration have been increasingly demanded in recent years, a so-called opto-electronic integrated circuit (OEIC) provided with a photo diode which is a light receiving element and other electronic elements such as a bipolar transistor, a resistance and a capacitance is being developed. It is demanded that a light receiving element characterized in its high receiving sensitivity, high speed and low noise, and a bipolar transistor characterized in its high speed and high performance be provided in the OEIC. As a recent trend, the commercialization of products such as Blu-ray Disc (BD) and HD-DVD, in which a blue semiconductor laser (wavelength of 405 nm) is used as a light source, has started in response to a demand for a larger capacity of the optical disc. Accordingly, the development of an OEIC which achieves a high speed and a high receiving sensitivity in a short wavelength region corresponding to the blue semiconductor laser is awaited.
Below is described a conventional optical semiconductor device.
FIG. 11 is a schematic sectional view of an optical semiconductor device (OEIC) having a conventional structure. In the example of the drawing is illustrated an OEIC provided with a silicon substrate as a semiconductor substrate, a double polysilicon emitter high-speed NPN transistor as a bipolar transistor and a pin photodiode as a light receiving element on a same substrate.
Referring to reference numerals shown therein, 1 denotes a low concentration p-type silicon substrate, 2 denotes a photodiode formed on the substrate 1, 3 denotes an NPN transistor formed on the silicon substrate, 4 denotes a high concentration p-type embedding layer formed on the silicon substrate 1, 5 denotes a low concentration p-type epitaxial layer formed on the p-type embedding layer 4, 6 denotes an n-type epitaxial layer formed on the p-type epitaxial layer 5, and 7 denotes a LOCOS isolation layer formed on the n-type epitaxial layer 6.
In the photodiode 2, 8 denotes a cathode layer made of the n-type epitaxial layer 6, 9 denotes a cathode contact layer formed on the cathode layer 8, 10 denotes a cathode electrode selectively formed on the cathode contact layer 9, 11 denotes a p-type anode embedding layer formed in the interface between the p-type epitaxial layer 5 and the n-type epitaxial layer 6, 12 denotes a p-type anode contact layer formed on the anode embedding layer 11, and 13 denotes an anode electrode formed on the anode contact layer 12.
In the NPN transistor 3, 14 denotes a high concentration n-type collector embedding layer formed in the interface between the p-type epitaxial layer 5 and the n-type epitaxial layer 6, 15 denotes a high concentration n-type collector contact layer selectively formed on the collector embedding layer 14, 16 denotes a collector electrode formed on the collector contact layer 15, 17 denotes a p-type base layer selectively formed in the n-type epitaxial layer 6 on the collector embedding layer 14, 18 denotes a base electrode connected to the base layer 17, 19 denotes a high concentration n-type emitter layer selectively formed on the base layer 17, and 20 denotes an emitter electrode formed on the emitter layer 19.
21 denotes a first insulation film formed on the n-type epitaxial layer 6, 22 denotes a second insulation film formed on the first insulation film 21, and 23 denotes a light receiving surface created in such a way that the second insulation film 22 of the photo diode 2 is selectively removed in order for the first insulation film 21 to be exposed. A thickness and a refractive index of the first insulation film 21 are optimized, so that a reflection preventing film for reducing the reflection of an incident light in the interface is provided.
An operation of the OEIC thus constituted is described below.
The light enters through the light receiving surface 23 and is absorbed by the cathode layer 8 and the p-type epitaxial layer 5 which is an anode. As a result, electron-hole pairs are generated. When a reverse bias is applied to the photo diode 2 at the time, a depletion layer extends on the side of the p-type epitaxial layer 5 of which the dopant concentration is low. Of the electron-hole pairs generated in the vicinity of the depletion layer, the electrons and the holes are diffused and drifted and thereby separated from each other, and arrive at the cathode contact layer 9 and the anode embedding layer 11, respectively. Then, carriers are retrieved as optical current from the cathode electrode 10 and the anode electrode 13. The optical current is amplified and signal-processed by an electronic circuit comprising the NPN transistor 3 and the resistance element and capacitance element provided on the silicon substrate 1, and then outputted as recording and reproduction signals for the optical disc.
In the structure according to the conventional technology, however, the optical current in the photodiode 2 is roughly divided into diffusion current components and drift current components. The diffusion current is dominated by the diffusion of minority carriers up to the end of the depletion layer. Therefore, a response speed of the diffusion current component is lower than that of the drift current component resulting from an electrical field in the depletion layer. Further, there are some carriers which are recombined before reaching the depletion layer. Thus, the diffusion current may cause the deterioration of a frequency characteristic and light receiving sensitivity of the photodiode 2. The percentage of the carriers absorbed in a surface vicinity is increased as the optical wavelength is shorter. In the case of silicon, for example, the depth of approximately 11 μm is necessary in order to obtain the carrier absorption ratio of 95% in the red light having the wavelength of 650 nm which is used as the light source for DVD, while the absorption ratio at the same level can be obtained in the depth of approximately 0.8 μm in the case of the blue light having the wavelength of 405 nm. Thus, a light having a short wavelength is seriously affected in the vicinity of the silicon surface.
Below is described another optical semiconductor device proposed in order to solve the problem. FIG. 12 is a schematic sectional view of the OEIC thus proposed.
In FIG. 12, 24 denotes a low concentration first p-type epitaxial layer formed on the high concentration p-type embedding layer 4, 25 denotes a low concentration second p-type epitaxial layer formed on the first p-type epitaxial layer 24. This constitution is different to that of FIG. 11 in that the reference numerals 24 and 25 both denote the p-type epitaxial layers. The rest of the constitution is the same as that of the conventional example illustrated in FIG. 11.
In this constitution, a cathode made of the cathode contact layer 9 and an anode made of the first p-type epitaxial layer 24 and the second p-type epitaxial layer 25 constitute the light receiving element. In comparison to the constitution illustrated in FIG. 11, the cathode layer is very thin.
When the light enters through the light receiving surface 23, it is absorbed by the cathode contact layer 9, first p-type epitaxial layer 24 and second p-type epitaxial layer 25. As a result, electron-hole pairs are generated. The electrons and the holes are diffused and drifted and thereby separated from each other, and arrive at the cathode contact layer 9 and the anode embedding layer 11, respectively. As a result, optical current is generated. In the case where the depth of the cathode contact layer 9 is at most 0.3 μm, and the concentrations of the first p-type epitaxial layer 24 and the second p-type epitaxial layer 25 are approximately 1×1014 cm−1, for example, an anode depletion layer is extended by approximately 10 μm, and most of the incident light having a wavelength shorter than 650 nm which is particularly used for DVD is absorbed in the depletion layer. In other words, the diffusion current components are reduced and the drift current components are dominant in the optical current. Therefore, a high-speed response of the photodiode 2 can be realized.    PATENT DOCUMENT 1: 2005-183722 of the Japanese Patent Applications Laid-Open (Page 5-6, FIG. 1)    PATENT DOCUMENT 2: 2001-284629 of the Japanese Patent Applications Laid-Open (Pages 7-8, FIGS. 1-2)